Conventionally, various kinds of methods of calculating displacement, such as torsion, bending, etc. of large movable bodies, such as ships and aircrafts, architectural objects, such as houses and buildings, and structural objects, such as bridges, are put in practical use. One of such methods is visual checking. Moreover, as Patent Document 1 discloses, a method of using a measurement by a laser beam exists. Furthermore, a method of using a strain gauge exists.